The present invention relates to an improved shotcrete formulation, for application to walls of mine tunnels and a wide range of other uses.
Shotcrete is a mortar or concrete product which is conveyed from delivery equipment through a hose, and projected pneumatically at high velocity onto a surface. It has been used since the early 1960""s to protect exposed rocks from degradation due to weathering and dedication, in addition to providing support to loosened rock blocks in broken or overstressed ground. (Earlier use of the finer aggregrate gunite gained acceptance as a sealant during the 1950""s.) In the late 1970""s and during the 1980""s, admixtures to the cement/aggregrate mix were introduced to improve its physical properties. These admixtures included accelerators, superplacticiizers, steel fibers and latterly microsilica (silica fume). A standard shotcrete formulation currently comprises cement, silica fume, aggregrates and barite.
There are two basic shotcreting technologies: the xe2x80x9cdryxe2x80x9d process, in which a mixture of cement, fine and course aggregrates is pneumatically conveyed through a nozzle to a delivery hose where water is added through a water ring to the essentially dry materials; and the xe2x80x9cwet process, in which the cement, aggregrates and water are mixed to a plastic consistency before being conveyed hydraulically to the nozzle where compressed air is added to project the wet material onto the surface.
Much of the early research into shotcrete was considered an extention of concrete technology, when mix proportions were found to be the most important aspects to be evaluated. It is generally agreed that a suitable base mix with a relatively high cement and sand content compared to conventional concrete are necessary to provide plenty of paste to act as a cushion for the larger aggregrate to impact and keep rebound to a minimum. High compressive strengths, typical of shotcrete products, are also a function of the high cement contents in combination with the low waterxe2x80x94cement ratios used: 0.3% by weight for dry shotcrete and 0.35 to 0.45% by weight for wet shotcrete.
The incorporation of silica fume to shotcrete has provided a number of benefits to both its plastic and hardened properties. Silica fume in plastic shotcrete reacts with the products of the cement/water hydration reaction to create additional cementing compounds. This process not only improves the compressive strength of the hardened product, but also increases the density. The benefits, discussed above, with regard to the use of microsilica additive to shortcrete have lead to wide industry acceptance of the product. Research carried out to evaluate the properties of a silica fume shotcrete have shown the best results are obtained when dosage rates of 8-15% silica fume by weight of cement are added to the mix. A conventional shotcrete composition is:
However, silica fume, which is a byproduct of steel production, has been determined to be a carcinogen. Use of shotcrete-containing silica fume has thus been prohibited by law in certain jurisdictions.
Another ingredient of current shotcrete formulations, barite, has been found to be opaque to x-rays and gamma rays, thus proving useful in shielding against radiation from uranium and other radioactive minerals. It also provides better pumping and adhesion qualities during application of shotcrete.
Prior art references of background interest to the present invention include U.S. Pat. No. 3,006,777 of Atkin, U.S. Pat. No. 2,992,175 of Borst, U.S. Pat. No. 1,576,730 of Harth, U.S. Pat. No. 5,494,513 of Fu et al (which describes the use of large amounts of zeolite in portland cement to improve the strength and aeration qualities of lightweight concrete products), U.S. Pat. No. 5,402,455 of Angelo et al, U.S. Pat. No. 6,166,390 of Quapp et al, U.S. Pat. No. 6,046,374 of Stephens, and U.S. Pat. No. 5,819,186 of Stephens. Canadian background references of interest include Patents Nos. 773,614 of Aslam et al and 765,765 of Jackson and Canadian Patent Applications 2,260,267 (which describes a shotcrete formulation with a very high barite concentration) of Toma et al, 2,341,206 of Vanvor, and 2,211,145 of Quapp et al. It is an object of the present invention to provide an alternative composition of shotcrete that does not have carcinogenic properties such as found in conventional silica fume-containing shotcrete, while at the same time providing similar advantages of adhesiveness to surfaces and good binding found in conventional silica fume shotcrete. It is a further object of the present invention to provide a shotcrete composition that has improved radiation shielding effects over conventional barite-containing shotcrete.
In accordance with the present invention there is provided a cementitious shotcrete composition comprising a blend of the following ingredients in the following percentages by weight:
In a preferred embodiment of the present invention, where the shotcrete is specifically applicable for use in shielding against radiation, the blended cememtitious shotcrete composition comprises the following ingredients in the following percentages by weight:
In practice, the cementitious shotcrete composition of the present invention is preferably blended with aggregrates in a ratio range of from about 1:1 to about 1:3 by weight.